Method of treating aluminum strip

ABSTRACT

The disclosure teaches a method for producing an improved surface on certain aluminum alloys. The process is characterized by applying certain organic compounds to aluminum strip, heating the coated strip to decompose said compound and generate atomic hydrogen, whereby said hydrogen is adsorbed and absorbed by the aluminum strip, cooling said strip, and physically altering the surface of said strip by applying at least one chemical compound thereto which reacts with said strip.

United States Patent 11 1 Michelson et al.

[ METHOD OF TREATING ALUMINUM STRIP [75] Inventors: Christian E.Michelson; Sheldon W.

Dean, Jr., both of Hamden, Conn.

[73] Assignee: Olin Corporation, New Haven,

Conn.

[22] Filed: July 15, 1971 [21] Appl. No.: 165,781

51 Int. Cl C23g 1/22, C23g 3/02 [58] Field of Search 156/21, 22, 23;117/49, 118; 148/627; 204/33 [5 6] References Cited UNITED STATESPATENTS 3,671,333 6/1972 Mosier 148/627 1451 Nov. 20, 1973 FOREIGNPATENTS OR APPLICATIONS 689,260 6/1964 Canada 117/118 PrimaryExaminerJacob l-l. Steinberg Att0rneyRobert H. Bachman et al.

[5 7 ABSTRACT The disclosure teaches a method for producing an improvedsurface on certain aluminum alloys. The process is characterized byapplying certain organic compounds to aluminum strip, heating the coatedstrip to decompose said compound and generate atomic hydrogen, wherebysaid hydrogen is adsorbed and absorbed by the aluminum strip, coolingsaid strip, and physically altering the surface of said strip byapplying at least one chemical compound thereto which reacts with saidstrip.

9 Claims, No Drawings METHOD OF TREATING ALUMINUM STRIP BACKGROUND OFTHE INVENTION The present invention relates to chemically treatedaluminum and aluminum alloys and specifically to aluminum or aluminumalloys having a pleasing or decorative surface thereon.

It is well known that aluminum is chosen for many applications becauseof its decorative appearance and excellent resistance to corrosion. Suchapplications include, for example, outdoor furniture, automotive trim,and railings. The aluminum, or alloys thereof, employed is generallychemically treated before use in order to provide a matt appearance,anodized to increase corrosion and wear resistance, or bright dipped inorder to provide a shiny and highly reflective surface. A frequentproblem, however, is that the aforementioned operations frequentlyresult in an uneven or streaked appearance due to surface imperfectionsin the metal or localized attack due to impurities or traces of oxidesin the surface, or attack at lattice imperfections such as vacancies andgrain boundaries.

It is therefore a principal object of the present invention to provide amethod of producing chemically treated aluminum which is uniform andpleasing to the eye.

It is a further object of the present invention to provide a methodwherein anodized, bright dipped, or etched aluminum is uniform inappearance.

It is a still further object of the present invention to provide amethod which is expeditious and convenient to use.

Further objects and advantages of the present invention will appearhereinafter.

SUMMARY OF THE INVENTION In accordance with the present invention it hasnow been found that the foregoing objects and advantages of the presentinvention may be readily obtained. The method of the present provides animproved surface on non-heat treatable aluminum or aluminum alloy strip.The method comprises: providing a material in strip form selected fromthe group consisting of non-heat treatable aluminum and aluminum alloys;coating said material with an organic compound which decomposes over 400F and which forms atomic hydrogen upon decomposition, preferably, saidcompound contains at least 14 carbon atoms; heating said coated materialto a metal temperature of at least 800 F for at least l minute todecompose said compound and generate atomic hydrogen whereby saidhydrogen is adsorbed and absorbed by said material, cooling saidmaterial, and physically altering the surface of said material byapplying at least one chemical compound thereto which reacts with saidmaterial, preferably chemically etching or bright dipping.

In accordance with the foregoing process, a chemically treated aluminummaterial is provided which is uniform and pleasing to the eye.Furthermore, an uneven or streaked appearance is substantiallyeliminated.

Further advantages of the present invention will appear from the ensuingdiscussion.

DETAILED DESCRlPTlON I The aluminum provided should be in strip form andmay be high purity aluminum or EC grade aluminum or generally anynon-heat treatable aluminum alloy. The present invention relates to thenon-heat treatable alloys, such as the American Aluminum Associationalloys of the following series: 1,000; 3,000; 4,000; 5,000; and 6,000.For example, the 1,000 alloy series covers high purity and commericalpurity aluminum. The 3,000 alloy series contains manganese as the majoralloying addition in an amount up to about 2 percent. The 4,000 alloyseries contains silicon as the major alloying addition in an amount upto about 15 percent. The 5,000 alloy series contains magnesium as themajor alloying addition in an amount up to about 8 percent. The 6,000alloy series contains magnesium plus silicon as the major alloyingadditions in an amount up to about 4 percent of magnesium plus silicon.Naturally, other alloying additions may readily be employed.

As indicated hereinabove, the aluminum should be provided in strip form.The strip may be cold worked, annealed, or partially annealed dependentupon particular requirements Furthermore, as indicated, the aluminumshould be a non-heat treatable material.

The aluminum is coated on a surface thereof with an organic compound ora mixture of organic compounds. Although only one surface of thematerial need be coated, all of the surfaces may be coated if desired.In those areas of the surface which are to be coated the coating shouldbe uniformly applied. Thickness of the coating is not critical so longas the surface is uniformly covered with the coating. Naturally, thethickness of the coating is dependent upon the method of application,such as by brush or cloth and also upon the viscosity of the particularcoating applied.

It is necessary that the particular organic compound or mixture thereofdecompose at a temperature above 400 F and not be highly volatile so asto vaporize away from the surface before the aluminum is heated totemperature; however, volatile carrier materials may be employed inaddition to the particular organic compound. It is also necessary thatthe organic compound or mixture liberate hydrogen upon decomposition.

It is preferred that the organic compound employed be a long chainedhydrocarbon having at least 14 carbon atoms and having no functionalgroups, such as an OH group. These compounds are not highly volatile andthus prior heating of the aluminum need not be employed beforeapplication. Heating of the aluminum prior to application, however, maybe necessary when more highly volatile compounds having fewer than 14carbon atoms are employed in order to minimize loss by volatilization ofthe compound during heat-up. Thus, ease of handling and preparation arefacilitated by using a compound having at least 14 carbon atoms.Representative organic compounds which may be employed in the presentinvention include hexadecane, the stearates, such as butyl stearate andmethyl stearate, and fatty acids such as stearic acid, oleic acid,lauric acid, etc. Esters, such as tri-butyl phosphate and tricetylphosphate and the acrylates such as butyl acrylate and methylmethacrylate, as well as the long chained polymers.

Following application of the aforementioned compounds to the aluminumstrip, the coated material is heated in a conventional furnace to atemperature of at least 800 F for at least 1 minute to decompose saidcompound and generate atomic hydrogen, whereby said hydrogen is adsorbedand absorbed by said material. Thus, the particular atmosphere in whichthe aluminum is heated is not critical and any suitable method may beemployed. Resistance or induction heating may also be readily employed,if desired.

Likewise the rate of heating is not critical so long as the aluminumreaches the temperature of 800 F and remains at temperature for at least1 minute. The maximum time at temperature is not critical, being limitedonly by practical considerations, and is generally about 1 hour.

The maximum temperature employed is also not critical although as apractical matter the upper temperature limit would be that of themelting point of the particular material employed, It is preferred,however, for the temperature to be from about 850 to l,050 F since below850 F the reaction becomes somewhat sluggish and above l,050 F thealuminum tends to soften and thereby distortion may occur. in addition,temperatures above l,0O F may result in excessive grain growth.

Following the heating step, the material is cooled to ambienttemperature and chemically treated in accordance with the presentinvention. The chemical treatment must be such as to physically alterthe surface of the aluminum strip by applying thereto at least onechemical compound which reacts with said aluminum strip. The chemicaltreatment may take the form of bright dipping, such as in a mixture ofnitric and phosphoric acids. The chemical treatment may also bechemically etching in a suitable alkaline solution, such as causticsoda.

in addition to the foregoing, after said chemical treatment the materialis preferably anodized in a suitable anodizing bath, such as chromicacid or sulfuric acid. In fact, it has been found that preferred resultsare obtained if the material is anodized after the chemical treatmentstep in order to obtain better corrosion and abrasion resistance andability to be colored, as well as other advantages.

In accordance with the present invention, it has been found that whenthe aluminum strip having an organic coating on its surface is heated toa temperature of at least 800 F for at least 1 minute, that hydrogen isproduced by the decomposition of the organic compound. The hydrogenproduced is atomic hydrogen and is adsorbed at the oxide surface,diffuses through the oxide and is absorbed by the aluminum, therebyentering the lattice of the alloy. A hydrogen atom present in thelattice of the aluminum tends to readily migrate to areas of highenergy, such as vacancies and grain boundaries. When the atomic hydrogenenters, for example, a vacancy, the energy of the vacancy is reduced toa level more nearly approximating that of the surrounding lattice. As'aresult of this, when the aluminum is chemically treated in accordancewith the present invention, a more uniform appearance is produced sincethe activity of lattice imperfections has been substantially reduced andthus selective attack at these high energy points does not occur. Thus,for example, upon subsequent bright dipping or chemical etching a moreuniform and pleasing surface is produced.

Such a treated material also produces a better and more corrosionresistant anodized film since likewise selective attack does not tend tooccur should the anodizing film be in some way penetrated by an outsidecorrosion, thereby resulting in localized attack at these high energypoints.

The present invention thus provides a method of producing an improvedsurface on aluminum and its alloys thereby resulting in greatlyincreased esthetic or decorative appeal.

The present invention is readily applicable to producing patterns indesired configurations on the surface of the material. This isaccomplished by coating the material with the aforementioned organiccompound only in those areas in which it is desired to increase theuniformity of the chemical treatment. Thus, the present invention issubject to a wide variety of uses and applications in which decorativepatterns are highly desirable. Such applications would include, forexample, home furnishings of various types, such as towel racks andshower curtain rods.

The present invention will be more readily apparent from the followingillustrative examples.

EXAMPLE I The present example illustrates the formation of hydrogen onan aluminum surface and on its entry into the metal.

A continuous length of Aluminum Alloy 5052 tubing was connected to asource of Geiger counting gas and a Geiger counting chamber. Part of thetube was painted with an organic ink in which part of the hydrogen atomswas replaced by tritium, a radioactive isotope of hydrogen. This paintedportion was introduced into a furnace at 930 F and the counting gasallowed to continuously sweep through the tube and then through thecounter. In about 5 minutes the counting rate was at least double normalbackground (35 d/m) and within about 20 minutes the rate was at least 20times background. This indicated the entry of the radioactive atoms intoand through the metal and finally into the counter volume.

EXAMPLE II The present example illustrates that hydrogen formed byreaction with aluminum and organic ink produces a physical nonuniformitybefore chemical treatment.

AA alloy 5252, 5005, 5052, 5086, and 5457 panels (0.030 inch thick) werepainted (pattern on one side) with the untritiated ink used in Example Iand heated for 30 minutes at 930 F. They were then removed from thefurnace and the ink pattern was clearly visible on both sides of thepanels. The side opposite the painted area did not have thecharacteristic light white stain characteristic of aluminum magnesiumalloys heated in this way.

EXAMPLE III The present example shows the effect of hydrogen in aluminumupon the uniformity of etching.

The alloys of Example ll were treated by the procedure in Example [I andthen immersed in a caustic etch from 1 minute to a time required toalmost completely dissolve the sample. During this entire period thepattern of original ink application was visible from both sides asevidenced by a more uniform and glossy appearance of the metal in thepattern area as contrasted with the uncoated area.

EXAMPLE IV The present example illustrates the effect of hydrogen inaluminum on bright dipping.

The alloys in Example II were painted (pattern on one side) with theuntritiated ink of Example I and heated for 30 minutes at 930 F. Thesamples were bright clipped at 200 F for 3 minutes in a bath containingapproximately 5% I-INO 85% H PO and percent water.

Substantially no pitting and grain boundary attack was found to haveoccurred in the area covered by the ink.

EXAMPLE V EXAMPLE VI The present example shows the effect of hydrogen inaluminum on bright dipping.

Example V was repeated except that the alloy samples were bright dippedat 200 F for 3 minutes in a bath containing approximately 5% HNO 85% l-IPQ, and 10 percent water, as in Example IV.

Substantially no pitting and grain boundary attack was found to haveoccurred in the area covered by the coating.

EXAMPLE VII The present example shows the effect of hydrogen in aluminumupon the uniformity of etching.

Example V was repeated employing butyl stearate as the coating and againa more uniform and glossy appearance was produced in the pattern area ascontrasted with the uncoated area.

EXAMPLE VIII Examples III and IV were repeated but before etching orbrightening, the panels were buffed. Again the ink pattern was clearlyvisible after the first minute of etching or brightening, even thoughthe original surface had been removed in the buffing operation.

The present example thus shows that the effect of hydrogen in aluminumis not caused by an initial surface oxide.

EXAMPLE IX In the present example mechanical working was employedfollowing coating and heating.

An alloy 5252 panel was treated as in Example II, then cold rolled 50percent. It was then etched as in Example III. The ink patternoriginally placed in a square grid pattern was visible as an elongatedgrid after etchmg.

It is thus seen that mechanical working following coating and heatingdoes not effect subsequent chemical treatment.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

l. A method of producing an improved surface on non-heat treatablealuminum or aluminum alloy strip comprising:

A. providing a material in strip form selected from the group consistingof non-heat treatable aluminum and aluminum alloys;

B. coating said material with an organic compound chosen from the groupconsisting of long chain hydrocarbon containing at least l4 carbonatoms, higher fatty acids and esters, long chain polymers, acrylateesters, and phosphate esters, which decomposes at a temperature of atleast 400 F and which liberates atomic hydrogen upon decomposition;

C. heating said coated material to a metal temperature of between 850and l,050 F for at least 1 minute to decompose said compound andgenerate atomic hydrogen, whereby said hydrogen is adsorbed and absorbedby said material and enters the lattice thereof;

D. cooling said material; and

E. physically altering the surface of said material by applying at leastone chemical compound thereto which reacts with said material.

2. A method according to claim 1 wherein said physical alteringcomprises treating the surface of said material in a manner selectedfrom the group consisting of chemical etching and bright dipping.

3. A method according to claim 2 wherein said physical altering is bychemical etching in a caustic bath.

4. A method according to claim 2 wherein said physical altering is bybright dipping in an acid bath.

5. A method according to claim 2 wherein said organic compound containsat least 14 carbon atoms.

6. A method according to claim 2 wherein said coating is selected fromthe group consisting of hexadecane, butyl stearate, stearic acid,tri-butyl phosphate and butyl acrylate.

7. A method according to claim 2 wherein said coating is applied toselected areas of said surface to form a pattern. I

8. A method according to claim 2 further including the step of anodizingsaid material following said physically altering.

9. A method according to claim 2 wherein said coating is applied in auniform layer to the surface of said material.

2. A method according to claim 1 wherein said physical alteringcomprises treating the surface of said material in a manner selectedfrom the group consisting of chemical etching and bright dipping.
 3. Amethod according to claim 2 wherein said physical altering is bychemical etching in a caustic bath.
 4. A method according to claim 2wherein said physical altering is by bright dipping in an acid bath. 5.A method according to claim 2 wherein said organic compound contains atleast 14 carbon atoms.
 6. A method according to claim 2 wherein saidcoating is selected from the group consisting of hexadecane, butylstearate, stearic acid, tri-butyl phosphate and butyl acrylate.
 7. Amethod according to claim 2 wherein said coating is applied to selectedareas of said surface to form a pattern.
 8. A method according to claim2 further including the step of anodizing said material following saidphysically altering.
 9. A method according to claim 2 wherein saidcoating is applied in a uniform layer to the surface of said material.